As a micro lens comprising a lens formed on a base material or a micro lens array comprising plural lenses formed on the base material, of an optical device, that is made of various materials and has various shapes, besides a micro lens made of an inorganic material such as glass, as a lens material, a micro lens made of resin has been proposed in terms of ease of manufacture and a cost advantage. According to the method of manufacturing the micro lens made of resin, because a lens-forming resin solution is placed on the base material and a hemispherical lens shape is formed due to its surface tension, a base material structure such that suppresses diffusion of the resin solution in the region other than the lens forming region is adopted.
For example, Prior Art 1 (Japanese Unexamined Patent Publication No. JP-A-2-262601) proposes a method comprising: preforming patterns made of a material expelling photosensitive resin as a resin composition for lens on the surface of a transparent base material, and forming lenses by dropping the resin composition for a lens within the patterns surrounded by the expelling portion. Prior Art 2 (Japanese Unexamined Patent Publication No. JP-A-4-190301) proposes a method comprising: forming a patterned masking member made of a sheet of nickel that is non-wettable for a liquid of resin for a lens on a base material, dropping a viscous lens-forming liquid on the surface of the patterns, forming the liquid into a spherical shape due to its surface tension, and forming lenses by solidifying the liquid.
Apart from the foregoing, Prior Art 3 (Japanese Unexamined Patent Publication No. JP-A-2001-141906) proposes a method comprising: forming plural micro lenses on a wettability-variable layer capable of varying wettability of a surface of a base material. According to this method, a photocatalyst-containing layer is formed first across the entire surface of the base material. Energy is then irradiated onto the photocatalyst-containing layer to form regions having different wettabilities in patterns. This forms the structure so as not to diffuse a resin solution to any region other than the lens-forming regions when a resin composition for lens is dropped onto the patterns.
Further, Prior Art 4 (International Publication No: WO95/9372) proposes a method comprising: providing lens-forming division lines on a base material having a contact angle of 15 degrees or higher with respect to lens-forming resin made of resin, such as phenol resin.
According to the foregoing methods for providing a layer of resin matrix to the region other than the lens-forming regions, however, the resin is merely attached to the base material with physical absorption, and thus the adhesion to the base material is weak, which readily gives rise to layer separation in the following treatment steps. It is therefore difficult to form fine lens patterns at a high degree of accuracy when the lens-forming resin is placed on the base material after the resin matrix is formed. In particular, in the case where the resin matrix is made of resist resin, the kinds of lens-forming resin or solutions to disperse the lens-forming resin are limited due to its easy separation, and the selections of the refractive index are also limited.
In addition, when the micro lens as described above is formed, it is preferable to reduce an optical path length to achieve a reduction of the device in size.
From the view point of a reduction of the optical path length, because the wettability-variable layer made of resin is provided on the base material and the lens-forming resin is placed on the layer according to the manufacturing method described in Prior Art 3, the optical path length becomes longer correspondingly to the resin thickness of the layer. Also, the optical design is required to take into account three materials of the base material, the wettability-variable layer and the lens-forming resin, each of which has a different refractive index.
Meanwhile, according to Prior Art 1, 2, or 4, openings are made on the base material by covering the base material with the resin layer only at specific regions, and the lens-forming resin solution is placed at these openings.
However, as is shown in FIG. 13, because a liquid of resin is used even in the case where two or more lenses are isolated by a resin matrix 101 of this kind having openings on a base material 1, the resin matrix 101 becomes as thick as several tens μm to several mm. Hence, when lenses 102 are formed, lens base portions 103 of a size comparable to the depth of the openings become dead volumes and the lens thickness is increased, which consequently makes it impossible to reduce the optical path length.
The problems discussed above become obstacles when fine micro lenses are formed at a high degree of accuracy. A reduction of the device in size requires not only to make the lenses per se finer, but also to lessen the lens thickness; however, it is difficult for the methods proposed in the related art to further lessen the thickness.
As means for solving the problem of the lens thickness as described above, there has been proposed a method for forming a thin-film monolayer on the base material instead of the resin matrix.
For example, Prior Art 5 (US Patent Publication No: US-A1-2005-31973) describes a pattern forming body comprising a base material, a photocatalyst-containing layer formed on the base material, a protection portion formed on the photocatalyst-containing layer, a property variable layer formed to cover both the photocatalyst-containing layer and the protection portion and whose surface property is varied by a function of a photocatalyst due to energy irradiation, and a property-varied pattern that is the property variable layer having its property been varied. The Prior Art 5 describes the application of the pattern forming body to a micro lens.
Also, Prior Art 6 (Patent Publication No: US-A1-2004-163758) describes a method of using a self-assembled patterned monolayer. According to this method, a stamp having a specific pattern with its surface being covered with organic molecules is formed first. Then, the organic molecules are transferred onto a base material by bringing the stamp into contact with the base material, whereby the self-assembled patterned monolayer is formed on the base material. The micro lenses are then formed by forming a thin film in spaces of patterns of this monolayer.
Further, Prior Art 7 (US Patent Publication No:US-A1-2004-151828) proposes a method comprising: forming a photoisomerization layer of a monolayer across the entire surface of the base material, and creating a gradient of surface tension by irradiating light to the photoisomerization layer so as to cause a lens-forming droplet dropped onto the photoisomerization layer to move a desired position.
According to the method described in Prior Art 5, however, the photocatalyst-containing layer is formed across the entire surface of the base material, and the protection portion and the property variable layer are further formed on the photocatalyst-containing layer. Hence, although the property variable layer on the upper layer is a thin film, the overall thickness becomes larger. In addition, because the multi-layer structure is adopted, not only the optical design becomes complex, but also the selections of lens materials are narrowed.
Also, according to Prior Art 6, the monolayer is formed by transferring the organic molecules onto the base material from the surface of the stamp by means of stamp printing. It is therefore necessary to have a large quantity of organic molecules adhere onto the stamp, which makes it difficult to form a monolayer having a uniform thickness. In addition, in the case where highly active organic molecules are used to fix the organic molecules onto the base material, the organic molecules readily react with one another on the stamp. A formed film is likely to be thicker since a large quantity of organic molecules which have lost the activity to bind with the base material are on the base material in a free state. The monolayer thus formed is not fixed sufficiently because of inactivated organic molecules and therefore readily separates from the base material, which makes the non-affinity unsatisfactory. In particular, during the formation of the micro lenses in Prior Art 6, since neither the organic molecules are adhered onto the stamp nor they are transferred onto the base material in an atmosphere to prevent inactivation of the organic molecules as described above, it is likely to be insufficient to form a monolayer having excellent non-affinity for the lens material uniformly.
Further, since the method described in Prior Art 7 requires a step of varying the surface energy of the monolayer after the droplets are placed, it is extremely difficult to control the lens shape to be uniform when a lens array is manufactured.
For any of the methods using the monolayer, it is difficult to place a lens material in a specific shape at a specific position on the base material as the lens is becoming finer. In other words, when a thin-film monolayer fixed onto the base material can be deposited uniformly, the lens thickness may be reduced in some cases; however, as the lens diameter becomes smaller, the lens material diffuses more readily from the specific region at which the lens is disposed. For this reason, problems, such as the occurrence of deformation of the lens shape and a failure to obtain a lens of a specific height become more noticeable.
Further, for a lens array comprising plural lenses formed on the base material, it is preferable to make density higher by narrowing intervals between adjacent lenses as much as possible in achieving efficient use of light. However, as the lens pitch becomes narrower, diffusion of the lens material that occurs when the lens material is placed as described above readily gives influences to the adjacent lenses. The influences between the adjacent lenses in close proximity to each other also make it difficult to obtain a uniform lens array.